The present invention relates to a new method for destroying high concentrations of nitric oxides in gas streams. These streams may be recycle streams from regenerable NO.sub.x scrubbers for flue gas applications, or they may be industrial waste streams such as those from nitrification processes in the chemical industry.
Flue gases resulting from the combustion of carbonaceous material typically contain substantial quantities of pollutants, including nitrogen oxides. These pollutants combine with other substances found in the atmosphere to produce serious environmental hazards, such as acid rain and smog. It is therefore desirable to remove these pollutants from flue gases before they are dispersed into the atmosphere. It is also desirable to decompose these pollutants into other substances not having the deleterious environmental consequences possessed by nitrogen oxides.
There are two types of methods by which the quantities of NO.sub.x dispersed from combustion systems may be reduced. One type, known in the art as combustion modification, requires control over the combustion reaction producing the pollutant. These techniques have generally achieved fifty to sixty percent reductions in NO.sub.x emissions from conventional combustion systems.
A specific type of combustion modification, known as reburning, has achieved reductions approaching seventy percent. Using this technique, a secondary fuel source as introduced downstream of the primary combustion zone in the combustor to achieve reductions of NO.sub.x. This technique, however, is disadvantageous in that it requires large amounts of secondary fuel to accomplish the reburning of NO.sub.x, and additionally requires downstream injection ports in the combustor to effectively control NO.sub.x levels in the effluent stream. This method is not effective when more than seventy percent removal of effluent NO.sub.x is required.
The second type of NO.sub.x removal methods is known as post-combustion cleanup, whereby the pollutant is removed downstream of its formation. These techniques are more complex and expensive than combustion modification techniques, but are useful when NO.sub.x reduction levels higher than seventy percent are necessary. In systems employing this technique, a dry scrubbing sorbent or an aqueous sorbent, such as an active metal chelate, are typically used to remove NO.sub.x and other pollutants. Certain systems, such as that proposed by Walker, U.S. Pat. No. 4,615,780, additionally incorporate the step of regenerating the sorbent and producing a concentrated stream of NO.sub.x which can be recycled as part of the combustion air. This concentrated NO.sub.x is destroyed in the combustor and elemental nitrogen and oxygen are produced. This system produces maximum reductions of 60 to 70 percent of the NO.sub.x produced by combustion, substantially less as compared to the present invention.
Another scrubber system, proposed by Harkness, et al, U.S. Pat. No. 4,612,175, uses active Fe(II)EDTA as the chelate to remove NO from flue gases simultaneously with the removal of SO.sub.2. This method, however, produces as an end product aqueous hydroxylamine disulfonate and other sulfonates in a sludge with the aqueous sorbent. These products in themselves create environmental hazards which render the process described by Harkness disadvantageous.
It should be noted that the source of the high concentration NO.sub.x stream may be, as described above, a recycle stream from a regenerable scrubber or an industrial waste gas stream, such as that from nitrification processes as used in the chemical industry, high temperature smelting plants, nitric acid manufacturing plants, and any other process where a high concentration of NO.sub.x is in the waste stream. The present invention relates to removal and destruction of NO.sub.x from such high NO.sub.x concentration waste gas streams.